*{
    box-sizing: border-box;
    padding: 0;
    margin: 0;
}

body{
    padding-top: 30px;
}

.demo-box{
    width: 900px;
    margin-left: auto;
    margin-right: auto;
    margin-bottom: 20px;
    border: 1px solid #ccc;
    padding: 5px;

}